Getting data from a table meeting some requirements - mysql

Question

I'm stuck in a SQL logic problem.
I have a table "games" with some data where player1_id and player2_id are user ids that played with each other on a checkers match:
Each game starts with a different ambient, already specified in another table (keyed by "game_id" column).
Assuming that everybody is online, an user can't replay the same game.
How can I select only users that wasn't played the games I didn't played yet to match with me?
An AJAX call will match two users that didn't played an specific game yet and put them to play.
No matter if the user was player1 or player2 , he can't repeat a game.
Thank you very much.
Tables:
game_match
+----+---------+------------+------------+
| id | game_id | player1_id | player2_id |
+----+---------+------------+------------+
| 1 | 1 | 16 | 17 |
| 2 | 1 | 18 | 23 |
| 3 | 1 | 19 | 21 |
| 4 | 1 | 20 | 22 |
| 5 | 2 | 20 | 17 |
| 6 | 2 | 16 | 18 |
| 7 | 2 | 19 | 23 |
| 8 | 1 | 25 | 15 |
+----+---------+------------+------------+
Users
+----+-----------+
| id | name |
+----+-----------+
| 17 | Donald |
| 18 | Margarida |
| 19 | Daisy |
| 20 | Mickey |
| 21 | Steve |
| 22 | Raul |
| 23 | Janis |
| 24 | Michael |
| 25 | Sergio |
| 26 | Bill |
| 27 | Alina |
| 28 | Alana |
| 29 | Harumi |
| 30 | Danielle |
| 31 | Lisa |
+----+-----------+
Thank you!

Answer 1

Assuming you have a table named games and a $userid, you can do this readily with not exists:
select g.*
from games g
where not exists (select 1
from game_match gm
where gm.game_id = g.game_id and gm.player1_id = $userid
) or
not exists (select 1
from game_match gm
where gm.game_id = g.game_id and gm.player2_id = $userid
) ;
Note that you can implement the same logic with or and one subquery. However, with the right indexes, two subqueries are more efficient.

Answer 2

To get your users I would use a union in a subquery:
select *
from users
where id not in
(select player1_id as played
from game_match
where player2_id = #logged_in_user
union
select player2_id
from game_match
where player1_id = #logged_in_user
) as played

Answer 3

Try the following solution using NOT EXISTS and INNER JOIN:
select name
from Users AS u
where not exists(
select 1
from game_match AS gm INNER JOIN
(select distinct game_id
from game_match
where #current_user_id = player1_id OR #current_user_id = player2_id) gids
USING(game_id)
where u.id = gm.player1_id OR u.id = gm.player2_id
);